Neoplasm (from ancient Greek νεο- neo-, "new" + πλάσμα plasma, "formation", "creation") is an abnormal mass of tissue as a result of neoplasia. Neoplasia is the abnormal proliferation of cells. Prior to neoplasia, cells often undergo an abnormal pattern of growth, such as metaplasia or dysplasia.[1] However, metaplasia or dysplasia do not always progress to neoplasia. The growth of neoplastic cells exceeds, and is not coordinated with, that of the normal tissues around it. The growth persists in the same excessive manner even after cessation of the stimuli. It usually causes a lump or tumor. Neoplasms may be benign, pre-malignant (carcinoma in situ) or malignant (cancer).

In modern medicine, the term tumor means a neoplasm that has formed a lump. In the past, the term tumor was used differently. Some neoplasms do not cause a lump.

Malignant neoplasms are commonly called cancer. They invade and destroy the surrounding tissue, may form metastases and eventually kill the host. Potentially malignant neoplasms include carcinoma in situ. They do not invade and destroy but, given enough time, will transform into a cancer.

Secondary neoplasm refers to any of a class of cancerous tumor that is either a metastatic offshoot of a primary tumor, or an apparently unrelated tumor that increases in frequency following certain cancer treatments such as chemotherapy or radiotherapy.

Because neoplasia includes very different diseases, it is difficult to find an all-encompassing definition.[3] The definition of the British oncologist R.A. Willis is widely cited: "A neoplasm is an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of the normal tissues, and persists in the same excessive manner after cessation of the stimulus which evoked the change."[4] This definition is criticized because some neoplasms, such as nevi, are not progressive.

Neoplastic tumors often contain more than one type of cell, but their initiation and continued growth is usually dependent on a single population of neoplastic cells. These cells are presumed to be clonal – that is, they are descended from a single progenitor cell.

Sometimes, the neoplastic cells all carry the same genetic or epigenetic anomaly that becomes evidence for clonality. For lymphoid neoplasms, e.g. lymphoma and leukemia, clonality is proven by the amplification of a single rearrangement of their immunoglobulin gene (for B cell lesions) or T-cell receptor gene (for T cell lesions). The demonstration of clonality is now considered to be necessary to identify a lymphoid cell proliferation as neoplastic.[5]

It is tempting to define neoplasms as clonal cellular proliferations but the demonstration of clonality is not always possible. Therefore, clonality is not required in the definition of neoplasia.

Tumor (Latin for swelling, one of the cardinal signs of inflammation) originally meant any form of swelling, neoplastic or not. Current English, however, both medical and non-medical, uses tumor as a synonym of neoplasm.[6]